U.S. Pat. No. 6,326,707 discloses linear actuators that are driven by shape memory alloy (SMA) materials, and feature stroke amplification by multiple bars or rods (sub-modules) linked together by SMA wires. In these and other SMA mechanisms, it has been understood that a restoring force is necessary to return an SMA wire from its heated (contracted) state to its cooled (extended) state. Many prior art SMA actuator designs have made use of common spring assemblies, such as helical or leaf springs, to exert the required restoring force. These spring assemblies typically deliver a spring force that varies linearly with displacement, (F=kx), and the restoring force in most cases is a maximum at maximum stroke. It has been found that the SMA component responds poorly to this force/displacement characteristic, and the useful life of the SMA actuator is severely limited by such a restoring force. The patent referenced above describes several spring arrangements that deliver variable restoring force (variable, or inverse linear, or the like) to optimize the performance of the SMA components.
It is apparently not widely known that some commercially available SMA wires, due to well-understood material processing steps, have the ability to return completely to their original shape without application of an external restoring force. This behavior is termed the reversible shape memory effect. The force produced as the wire cools and returns to its quiescent length is very small; that is, a small fraction of the useful force produced when it contracts upon heating. It is not practical to make a device that produces usable force on the return stroke as well as the forward stroke. One reversible shape memory device in the prior art is a helical spring that expands lengthwise upon heating, and contracts fully to its quiescent length upon cooling. There appears to be no other devices in the prior art that exploit the reversible shape memory effect to useful effect.
The present invention generally comprises a linear actuator that employs a shape memory alloy component as the driving element. One salient aspect of the invention is that it introduces an Intrinsic Return Means (IRM) to the SMA linear actuator, thereby obviating the use of a spring return mechanism or the like. Another significant aspect of the invention is that it introduces stroke amplification by multiple segments in a rotational actuator. A further significant aspect is the introduction of a simplified linear actuator assembly.
In general, the most fundamental aspect of IRM is the use of SMA wire that is known to exhibit reversible shape memory effect, and structural means for confining or constraining the wire to move solely along a defined line or curve as it contracts and relaxes. The structural means provides a low friction guide to direct the wire. Given the fact that the reversible shape memory effect will cause the wire to elongate upon cooling to substantially 100% of the original length, it necessarily follows that the low friction guide will cause the wire to return to its original, quiescent configuration. The guide (such as a groove or channel or tube) may be linear, and may be curved if the radius of curvature is much greater than the diameter of the SMA wire.
In a rotational embodiment of the concept described above, a cylindrical bobbin is provided with one or more turns of a helical groove formed in the outer peripheral surface of the bobbin. A SMA wire extends from a mechanical ground to the helical groove to wrap about the bobbin. A bobbin cover, comprising a cylindrical tubular sleeve having a grooved inner surface formed to complement the helical groove of the bobbin. The confronting grooves of the bobbin and cover define opposed sides of a continuous channel that contains and constrains the wire to expand and contract longitudinally along the channel, thus ensuring that the wire will re-assume its original, quiescent configuration when it cools below its transition temperature. A number of turns may be placed in a small length of bobbin, due to the small diameter d of the SMA wire compared to the bobbin diameter D (D≈100d), whereby the rotational excursion of the bobbin may be increased by each additional turn of the SMA wire.
The SMA wire is connected at opposite ends to the fixed bobbin cover and the rotatable bobbin. The rotating bobbin may be coupled to a machine that does useful work upon rotation, such as an iris mechanism used in a fluid flow valve or camera exposure control, and the like. Electronic control of the current through (and thus the temperature of) the SMA wire enables precise control of the contraction of the SMA wire and thus of the angular excursion of the bobbin with respect to the mechanical ground. Note that the bobbin and cover assembly requires a small axial dimension to incorporate a number of turns of wire and has a relatively small peripheral thickness (outer diameter minus inner diameter).
In a further rotational actuator embodiment, a plurality of narrow, coaxial rings are provided, the rings being nested in close concentric fit. Each ring is provided with a groove extending about the outer (or inner) peripheral surface thereof, the confronting grooves of the multiple rings forming opposed sides of annular channels. A plurality of SMA wires is provided, each wire secured at one end to one ring and extending to wrap about the adjacent inner ring. (Alternatively, a single SMA wire may extend about each ring and pass through to the next ring.) The wires are electrically connected for ohmic heating, whereby contraction of the wires causes each ring to rotate with respect to the adjacent inner ring. The wires may be activated as a group for full rotation, or individually for incremental rotation of the inner element. The rotation of the rings is additive, as in the stroke amplification mechanisms of U.S. Pat. No. 6,326,707, whereby the outer ring may be fixed and the inner ring may undergo a significant angular excursion. (Note that the construction may be reversed so that the inner ring may be fixed and the outermost ring undergoes the additive rotations of the plurality of rings.) The rings are narrow and thin, and form an assembly that occupies very little space in the axial or radial dimensions.
In another embodiment for rotational actuation, a plurality of narrow rings are disposed in stacked, adjacent relationship. Extending axially from each ring is a pin than protrudes through a slot formed in the adjacent ring. A plurality of SMA wires is provided, each secured at one end to the pin anchored to the respective ring, and secured at the other end to the pin projecting through its slot from the adjacent ring. (Alternatively, a single SMA wire may extend about each ring and pass through to the next ring.) Each wire is received in an annular peripheral groove extending about its respective ring, and extends thereabout at least one turn. Ohmic heating contracts the wires, which rotate the rings in additive fashion in the same direction. A sleeve member may be received about the stacked rings to impinge on the plurality of wires in their grooves and constrain and confine the wires to achieve the intrinsic return effect described above.
In an embodiment for linear actuation, the invention provides a bar-like component having top and bottom surfaces, and opposed ends spaced apart longitudinally. A pair of crimp recess holes extend from the top through to the bottom surface, each hole disposed adjacent to a respective end of the bar. A pair of longitudinal grooves extend between the crimp recess holes, each groove formed on a respective top or bottom surface.
Two or more bar components may be stacked together, the top surface of one bar impinging on the bottom surface of the superjacent bar in the stack. An SMA wire having a lug crimped at each end is disposed between adjacent bar components in the stack. The wire is received in the aligned grooves of the top and bottom surfaces of adjacent bar components, One crimped end of each wire is received in the crimp recess of one bar component, and the other crimped end is received in the crimp recess of the opposed end of the superjacent bar component. The wire is constrained and confined within the aligned grooves of each pair of bars in the stack. Each wire may be heated to cause contraction and translate each bar with respect to its superjacent counterpart. The translation is amplified by the additive effect of the linked bar components. In addition, the SMA wires are restricted to longitudinal movement within the channel formed by the first and second grooves to achieve the intrinsic return effect.
The invention includes a lost motion coupling to join two counter-acting SMA stroke amplification devices, whether linear or rotational. The coupling enables the two devices to drive an actuating member reciprocally, each device extending and resetting the other when fully extended.
Although the invention is described with reference to the shape memory component comprising a wire formed of Nitinol, it is intended to encompass any shape memory material in any form that is consonant with the structure and concept of the invention.